use send rather than ssend to avoid lockup

[dstndstn]

Hi,

I'm using Ubuntu 20.04, with the OS openmpi package, mpi4py 3.1.5, and schwimmbad 0.3.2. This is on the "symmetry" cluster at Perimeter Institute.

The behavior I'm seeing is that when creating an MPIPool(), I see each worker getting one task, it finishes the task and sends the result back, and the boss receives the result, but the workers never proceed to the next task.

Via some sophisticated printf debugging, I found that the workers were never returning from the self.comm.ssend() call. My wise colleague suggested changing that to self.comm.send(), and then it works perfectly!

I don't think you need any of the synchronization implied by ssend, so this should be fine?

My system details:

$ mpiexec --version
mpiexec (OpenRTE) 4.0.3
$ ls -l $(which mpiexec)
lrwxrwxrwx 1 root root 25 Aug 15 2023 /usr/bin/mpiexec -> /etc/alternatives/mpiexec
$ ls -l /etc/alternatives/mpiexec
lrwxrwxrwx 1 root root 24 Aug 15 2023 /etc/alternatives/mpiexec -> /usr/bin/mpiexec.openmpi

[adrn]

Hey! I'm just getting back to work from parental leave, but I'll take a look at this within the next few weeks. Thanks for this!

[adrn]

Whoops, where did those months go? Thanks for the patience -- I haven't seen the issue you described, but I also don't know why this was using ssend to begin with (it probably traces back to ye olde MPIPool implementation in emcee, where some of this all started...). So I'm find with changing it to the more standard send! Thanks for catching.